The present invention relates to a method and a device for vibration control, and more specifically a method and a device for vibration control in internal turning, and a tool holder for vibration control in internal turning.
In internal turning, dynamic motion arises between the tool and the workpiece. The motion is largely due to the fact that the chip-forming process, i.e. the removal of the generally relatively hard material from the workpiece, results in dynamic excitation of the tool, especially the tool holder. The dynamic excitation results in a dynamic motion, in the form of, for instance, elastic bending or torsion, of the tool and the tool holder. The chip-forming process is largely stochastic and the excitation results in tool vibrations and noise. In addition to thus causing problems in the working environment, the dynamic motion also affects the evenness of the surface of the workpiece and the service life of the tool.
It is therefore important to reduce the dynamic motion as far as possible. It has been known for long that the vibration problem is closely connected with the dynamic stiffness in the construction of the machine and the material of the workpiece. It has therefore been possible to reduce the problem to some extent by designing the construction of the machine in a manner that increases the dynamic stiffness. Moreover, it has recently been possible to increase the dynamic stiffness of the tool itself and the tool holder by active methods for controlling the response of the tool. This means that active control of the tool vibrations is applied.
The active control comprises the introduction of secondary vibrations, or countervibrations, in the tool by means of a secondary source which is called actuator. The actuator is operated in such manner that the control vibrations interfere destructively with the tool vibrations.
In internal turning, the tool is affected by excitation forces in the cutting speed direction, i.e. the direction of rotation of the workpiece at the cutting edge of the tool, in the direction of feed, i.e. axially seen from the perspective of the workpiece, and in the radial direction, i.e. radially seen from the perspective of the workpiece. The radial direction thus is perpendicular to the cutting speed direction. There are no known solutions for reducing tool vibrations in internal turning. However, attempts have been made to solve the corresponding problem in external turning. The excitation forces in external turning correspond approximately to the excitation forces in internal turning, but there are essential differences in the response of the tool holders since their design differs.
U.S. Pat. No. 4,409,659 discloses an example of active control of the tool vibrations in external turning. An ultrasonic actuator is arranged on the tool and produces countervibrations in the tool. The operating current of the actuator is controlled according to physical parameters that are measured and by means of the work of the actuator are kept within defined limits. This construction is unwieldy since the actuator is a comparatively large component which must be mounted on a suitable surface of the tool. Moreover, the directive efficiency of an ultrasonic actuator is not quite distinct.
JP-63,180,401 discloses a very different solution in external turning, where the actuator is built into the tool holder which holds a turning insert. A laterally extending through hole, which is rectangular in cross-section is formed in the tool holder. A piezoelectric actuator, in series with a load detector, is fixed between the walls that define the hole in the longitudinal direction of the tool holder. The load detector detects the vibrations and is used by a control unit to generate, via the actuator, countervibrations which reduce the dynamic motion. This construction necessitates a considerable modification of the tool holder and indicates at the same time that the designer has not been aware of the essence of the excitation process. In fact, the modification counteracts the purpose of the construction by reducing the stiffness of the tool holder in the most important directions, above all vertically, which in itself causes a greater vibration problem, or alternatively means that the dimensions of the tool holder must be increased significantly in order to maintain the stiffness. During external turning, the rotating tool produces a downwardly directed force on the cutting edge. When the cutting edge offers resistance, material is broken away from the workpiece. In this context, most of the vibrations arise. In JP-63,180,401, one imagines that the surface of the workpiece is uneven (wave-like) and thus mainly excites the tool holder in its longitudinal direction. Via the actuator, one generates an oscillation in opposition towards the wave pattern and thus obtains a constant cutting depth.
There is thus a need for a solution which controls the most essential vibrations, which is intended for internal turning, or drilling turning, and which causes a minimum of negative effects, such as bulky projections of dynamically weakening modifications, and still has a good effect.
An object of the present invention is to provide a well-functioning method and a well-functioning device for vibration control in internal turning.
The object is achieved by a device and a method according to the present invention. Thus, there is provided a device for vibration control in a machine for internal turning, said machine comprising a cutting tool supported by a tool holder, the device comprising a control unit, a vibration sensor connectable to the control unit, and an actuator connectable to the control unit. The actuator comprises an active element, which converts an A.C. voltage supplied by the control unit to the actuator into dimensional changes, wherein said active element is adapted to be embedded in the body of the tool holder. The active element is adapted to be embedded in such manner that said dimensional changes impart bending to the body of the tool holder.
Further, there is provided a method for vibration control in internal turning, comprising the steps of detecting the vibrations of a tool holder during working, and generating control vibrations in the tool holder, according to the detected vibrations and by means of at least one active element which is electrically controllable to dimensional changes. The method comprises the further steps of embedding said active element in the body of the tool holder and, for generating the control vibrations, imparting bending to the body of the tool holder by generating at least one control voltage and applying the control voltage across said active element,
Another object of the present invention is to provide a tool holder arranged for vibration control.
The object is achieved by a tool holder for internal turning, the tool holder comprising an actuator, said actuator comprising an active element which is electrically controllable to dimensional changes, wherein said active element is embedded in the body of the tool holder and is adapted to impart, through said dimensional changes, bending to the body of the tool holder.
The idea of embedding, according to the invention, at least one active element in the tool holder implies a minimal modification of the tool holder and at the same time uses the rapidity and the capability of changing dimensions of the active element in an optimal manner. The embedding is also advantageous by the device being useable in practice since it is protected against cutting fluids and chips. In addition to the prior-art devices not being designed for internal turning, they are designed in a manner which possibly makes them useable for laboratories, but not in the industry.
The device according to the invention is further adapted to impart bending to the tool holder through the arrangement of the active element/elements. The corresponding actuator element in JP-63,180,401 is deliberately arranged so that the dimensional change occurs along the longitudinal axis of the tool holder, which does not result in bending. This depends on an incomplete idea of what primarily causes the vibration problems. Thus one has not realised that the most important excitation forces have any other direction but parallel with the longitudinal axis of the tool holder. Even with this knowledge, the construction according to JP-63,180,401, however, is not easily adjustable to any other kind of mounting than the one shown.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.